1. Field of the Invention
The present invention relates to the use of a novel class of crystalline microporous metal aluminophosphates as catalysts. More particularly the invention relates to novel magnesium aluminophosphates, zinc aluminophosphates, cobalt aluminophosphates and manganese aluminophosphates. These compositions are prepared hydrothermally from gels containing reactive compounds of phosphorus, aluminum, at least one of the group magnesium, manganese, cobalt and zinc, and organic templating agents which function in part to determine the course of the crystallization mechanism and hence the structure of the crystalline product.
2. Description of the Prior Art
Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art and now comprise over 150 species of both naturally occurring and synthetic compositions. In general the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions which are not zeolitic, i.e. do not contain AlO.sub.2 tetrahedra as essential framework constituents, but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites are also known. Metal organosilicates which are said to possess ion-exchange properties, have uniform pores and be capable of reversibly adsorbing molecules having molecular diameters of about 6 A or less are reported in U.S. Pat. No. 3,941,871 issued Mar. 2, 1976 to Dwyer et al. A pure silica polymorph, silicalite, having molecular sieving properties and a neutral framework containing neither cations nor cation sites is disclosed in U.S. Pat. No. 4,061,724 issued Dec. 6, 1977 to R. W. Grose et al.
A recently reported class of microporous compositions and the first framework oxide molecular sieves synthesized without silica, are the crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440 issued Jan. 12, 1982 to Wilson et al. These materials are formed from AlO.sub.2 and PO.sub.2 tetrahedra and have electrovalently neutral frameworks as in the case of silica polymorphs. Unlike the silica molecular sieve, silicalite, which is hydrophobic due to the absence of extra-structural cations, the aluminophosphate molecular sieves are moderately hydrophilic, apparently due to the difference in electronegativity between aluminum and phosphorus. Their intracrystalline pore volumes and pore diameters are comparable to those known for zeolites and silica molecular sieves.
In copending and commonly assigned application Ser. No. 400,438, filed July 26, 1982, there is described a novel class of silicon-substituted aluminophosphates which are both microporous and crystalline. The materials have a three-dimensional crystal framework of PO.sub.2.sup.+, AlO.sub.2.sup.- and SiO.sub.2 tetrahedral units and, exclusive of any alkali metal or calcium which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system: "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular silicoaluminophosphate species involved; "x", "y", and "z" represent the mole fractions of silicon, aluminum and phosphorous, respectively, present as tetrahedral oxides. The minimum value for each of "x", "y", and "z" is 0.01 and preferably 0.02. The maximum value for "x" is 0.98; for "y" is 0.60; and for "z" is 0.52. These silicoaluminophosphates exhibit several physical and chemical properties which are characteristic of aluminosilicate zeolites and aluminophosphates.
Although a number of mineral species exist which contain either zinc, magnesium, manganese or cobalt in conjunction with aluminum and phosphorus, only the zinc-containing mineral kehoeite has a microporous crystalline structure and exhibits molecular sieving and ion-exchange properties. While it has been proposed that kehoeite has an analcime-type structure, [D. McConnell, Mineral Mag., Vol. 33, pg. 799 (1964)] the evidence is not conclusive due to poor agreement between X-ray powder diffraction pattern data, and the apparent necessity for a highly defective framework for kehoeite in order to match the unit cell content of analcime. In any event, zinc is not purported to be a framework constituent, but is present as a charge-balancing cation for the framework of linked AlO.sub.2.sup.-, H.sub.3 O.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral units.
Crystalline ternary aluminum phosphates and the method for their preparation are reported by Otto Schmitz-Dumont and Werner Hoffman in Z. Anorg. u. Allgem. Chem. 302, 121-135 (1959). Of the three structural types synthesized, only types (II) and (III) contain magnesium, cobalt or zinc, and these are found to lose water of hydration only at substantially elevated temperatures (350.degree. C.) and to become amorphous prior to or upon complete dehydration. Thus these materials are unlike the materials of the present invention which can be completely and reversibly dehydrated without becoming amorphous.